In recent years, attempts have been made to accurately keep track of a load (ground contact force) applied to a tire of a traveling vehicle. For example, in Patent Literature 1, a sensor unit having a gauge attached to a column member is buried in a road surface, and the sensor unit is configured to measure three-dimensionally the load when the tire passes. A plurality of the sensor units is arranged in a row in a direction perpendicular to the traveling direction of the tire, so that a load applied to the tire can be measured at a plurality of positions in the width direction of the tire. According to circumstances, a plurality of rows of the sensor units is arranged, so as to measure distribution and fluctuation of the load even in the traveling direction of the tire.